1. Field of the Invention
The field of the invention relates to bar code reading at point of sale (POS) locations. In particular the field of the present invention relates to a low-cost laser scanning technique using resonant cantilever beam deflectors for multiplexing scanning and collecting light beams. The invention makes possible a laser scanner which is small, compact, and efficient.
2. The Prior Art
In conventional applications the scanner is mounted in the counter of the POS station. The clerk moves the labeled goods over the window of the scanner and a laser beam within the scanner continuously scans a volume in space above the window. The checker or salesperson moves the package through this space with the label facing the window. When a laser beam crosses the black and White bars, the intensity variations in the scattered light are detected by a photodetector within the scanner. These are then converted to electrical signals which are decoded into the information on the label.
3. BACKGROUND OF THE INVENTION
Normally the scanning is accomplished using a motor which drives a rotor on which are mounted a number of mirrors. Such scanners are described in U.S. Pat. Nos. 4,064,390, 4,699,447, 4,799,164 and many others.
A further method of scanning uses a rotating hologram as in U.S. Pat. Nos. 4,795,224 and 4,647,143. This family of scanners also uses motors and requires considerable space for the hologram and the motor.
Alternatively, bar code labels are scanned using a handheld scanner in POS stations. This type of scanner may use a small mirrored polygon like the in-counter scanners, or it may use other techniques to scan the laser beam. One method uses a galvanometer driven mirror to scan the beam in a single line.
Another conventional technique uses a stepper motor to rotate the mirror back and forth. The advantage of this is that there is less dead time compared to a polygon scanner and to a sinusoidal galvanometer scanner. In addition, less space is required. Such a scanner is described in U.S. Pat. No. 4,825,057. Another disadvantage of the reversing stepper motor design is that a significant time delay is required to stop and start the mirror. In addition, this design generally uses more power than other motor designs.
Resonant mechanical scanners have been introduced in an attempt to overcome the disadvantages inherent in motor driven scanners. Resonant mechanical scanners are employed in a wide variety of applications. They are used because of their high reliability, long life and low-power consumption. Sinusoidally oscillated mechanical scanners have minimum energization requirements and are mechanically and electronically simple in design, fabrication and operation.
Resonant mirror systems for scanning laser beams in bar code reading are well known in the industry. An improved low profile scanner is described in U.S. Pat. No. 4,732,440. The main disadvantage of conventional resonant scanners is that the beam scan is sinusoidal, resulting in a velocity variation which goes from zero to some maximum. This variation is a serious problem for scanning labels because the scan pattern is unusable during the period of low velocity. U.S. Pat. No. 4,859,846 describes a dual-mode resonant scanning system designed to reduce the velocity variation by making the oscillation triangular through the addition of a third harmonic resonance. However, this method also is not well adapted to scanning the parallel, perpendicular bars of the standard bar code label.